I keep 12 blacksmiths permanently for quests, whether they are for events, DCs, story quests, etc. Yes, it is wasted space other than for questing, but then I don't have to keep deleting, adding, or rearranging things to fit them in. I find 12 to be a great number for fulfilling the variety of production options that typically come up in quests.
The reality is that most quests are unnecessary and inefficient, merely checking off boxes to get rewards. It's better to simply accept this reality and designate a small section of the city to complete these for rewards. Just think of the lost tile production as the cost to gain more efficient production elsewhere.